[Elecraft] K3: 6M SSB audio hash

Sun Jun 8 20:33:15 EDT 2008

Jim, 

> My point is that what you have described is ALSO WRONG. That 
> short jumper can cause common impedance coupling, as the voltage 
> drop across it is added to circuit common. That "short jumper" 
> has inductance, and at some frequency, and/or at some power level, 
> the voltage drop across it becomes strong enough to cause RFI. 

I'm aware that a short jumper is "wrong" from the purist point 
of view.  However, it is much better than mic return that is 
held above ground by an RF choke!   Most of the rigs I've seen 
have relatively good grounding for the "PTT ground" - the jumper 
is 1 cm or less or the circuit board layout includes the "PTT 
Ground" pin in a wide (large) ground plane.  

Fortunately, most amateur equipment is not operated in strong 
(multi hundred KW ERP) UHF fields.   

Yes, the best connection would be to tin the mic cable braid and 
clamp it and jumpers to both the "PTT ground" and "Mic ground" 
pins in the cable clamp of the Foster plug.  Unfortunately, the 
manufacturers have not chosen to do so.  

> On my website are photographs of very high quality (and very 
> expensive, German-made) condenser mics with serious RFI problems.

Which article?  I used many of the Neuman and AKG mics 30 years ago 
in the recording studio.  Of course, the "purists" were not about to 
give up their Nuvistor preamplifiers <G>. 

> One piece of good news -- the front panel mic connector on the K3 
> IS mounted directly to the shielding enclosure. So are all the RF 
> connectors. 

Unfortunately, neither the front panel nor the KIO3 have good 
shielding connections to the rest of the radio - at least not 
with my factory assembled K3.  I measure between 5.5 and 6.0 Ohms 
from the shell/ring of the Foster jack to the grounding lug and RF 
(antenna) connectors on the rear panel.  I also measure between 
5.5 and 6.0 Ohms from the sleeve of the rear panel (3.2 mm) Mic 
jack to the ground terminal and about 24 Ohms from the sleeve of 
the Line in and Line Out jacks to the ground terminal.  

73, 

   ... Joe, W4TV 

> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Brown
> Sent: Sunday, June 08, 2008 4:17 PM
> To: Elecraft List
> Subject: RE: [Elecraft] K3: 6M SSB audio hash
> 
> 
> On Sun, 8 Jun 2008 14:28:45 -0400, Joe Subich, W4TV wrote:
> 
> >Then I should not have said "PTT Ground" but that is how Icom, 
> >Kenwood and Yaesu all label their mic connectors.  In all three 
> >brands, the "PTT ground" is DC return and is, in fact, chassis 
> >ground - usually a short jumper from the "PTT Ground" pin to 
> >the chassis or the ground trace on a circuit board which contains 
> >mic connector which is tied to chassis with multiple grounding 
> >and mounting screws. 
> 
> Joe, 
> 
> Thanks for hanging in on this issue, because it is VERY important.
> 
> My point is that what you have described is ALSO WRONG. That 
> short jumper 
> can cause common impedance coupling, as the voltage drop across it is 
> added to circuit common. That "short jumper" has inductance, 
> and at some 
> frequency, and/or at some power level, the voltage drop 
> across it becomes 
> strong enough to cause RFI. 
> 
> On my website are photographs of very high quality (and very 
> expensive, 
> German-made) condenser mics with serious RFI problems. In one 
> of them, 
> the cable shield goes to the shell of the mic with a jumper 
> less than one 
> inch long. In downtown Chicago, where TV transmitters are on tall 
> buildings, that mic begins to detect TV broadcast stations at 
> roughly 180 
> MHz. An older mic from the same manufacturer begins detecting at TV 
> channel 2 (54-60 MHz) and FM broadcast!  Mics with shorter 
> jumpers begin 
> having trouble only on higher UHF channels. They ALL have 
> trouble with 
> cell phones. Documentation of this, along with the extensive 
> testing I've 
> performed, are on my website. 
> 
> http://audiosystemsgroup.com/publish
> 
> The voltage drop across that inductance is proportional to frequency, 
> inversely proportional to distance between the victim 
> equipment and the 
> transmitting antenna, and proportional to the square root of the 
> transmitter power!  
> 
> I will keep repeating this until it becomes clear. The ONLY 
> good place to 
> terminate a cable shield is to the shielding enclosure DIRECTLY. Any 
> other connection sets up the possibility of common impedance 
> coupling -- 
> what pro audio folks call "the pin 1 problem."  
> 
> >The whole point is that all of the "big three" manufacturers use 
> >the shield in their microphones improperly by connecting it to an 
> >ungrounded mic return instead of the chassis ("PTT Ground").  In 
> >the schematics I have checked, every one provides a DC return for 
> >the mic/preamp using an RF choke but connects the mic shield to 
> >the mic return instead of the chassis.  
> 
> I also see this, and I strongly agree with you that it is 
> VERY WRONG, and 
> is often the cause of RFI into the ham rig (what we call RF 
> feedback). 
> This improper connection of cable shields is the primary 
> cause of RFI of 
> all types, including RF in the shack. Add to this the US 
> members of that 
> group, which should more properly be called the big five. The 
> K2 and the 
> K3 have improperly terminated shields at some connectors. So 
> does my Ten 
> Tec Omni V. 
> 
> One piece of good news -- the front panel mic connector on the K3 IS 
> mounted directly to the shielding enclosure. So are all the RF 
> connectors. 
> 
> n Sun, 8 Jun 2008 09:39:20 -0700, Brian Lloyd wrote:
> 
> >I guess that people forget that, in shielding equipment, they are  
> >building a Faraday cage around it. That means that you need to  
> >terminate your shield at the OUTSIDE of the equipment, not 
> inside. One  
> >wants to continue the Faraday cage all the way out to the input  
> >device. This means that the shield of any wire needs to be 
> attached to  
> >the chassis externally. That isn't hard to understand. I 
> know that I  
> >solved the problem in my designs by using shielded twisted-pair for  
> >phono cartridge input and tying the shield to the chassis.
> 
> YES! 
> 
> 73,
> 
> Jim Brown K9YC
> 
> 
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